In construction vehicles of either the on or off-highway type, the vehicle engine is typicaly coupled with the traction wheels through a fluid torque converter and a change speed transmission. The fluid torque converter provides torque multiplication which is required when the vehicle is in a working mode, e.g. a front-end loader filling its bucket. When the vehicle is operated in a travel mode, the torque multiplication is not needed and, if it remains in the driveline, the torque converter operates with slippage and introduces power loss. To avoid this condition, it is common practice to provide the torque converter with a lockup clutch which bypasses the torque converter and providea d direct mechanical connection between the engine and the transmission. A typical lockup clutch is hydraulically actuated under the control of a solenoid valve.
The torque converter lockup clutch, as most commonly used in the prior art, is manually controlled by the vehicle operator. Converter lockup must be used selectively and the degree of effectiveness and efficiency of vehicle operation depends upon the judgment of the operator. For example, the torque converter should be operated in lockup under certain travel conditions at the work site or when the vehicle is on the highway between work sites; it should be operated in the unlock condition when the vehicle is in a work mode. It is difficult for the operator to initiate change between lockup and unlock conditions at the optimum converter speed to achieve the most efficient operation. Further, there are certain operating conditions in which proper use of converter lockup contributes to safety in vehicle operation and the avoidance of unnecessary wear or damage to the vehicle driveline.
A prior art converter lockup control system for a construction vehicle is shown in my U.S. Pat. No. 4,148,231. In that control sytem, manual control is provided for selective lockup and unlock by the vehicle operator. Further, that system includes automatic control for momentary unlock of the torque converter during each upshift and downshift above a certain speed range. There are other patents, such as Heino U.S. Pat. No. 4,208,929, Schneider et al. U.S. Pat. No. 3,805,640 and Akeson et al. U.S. Pat. No. 4,015,488 which disclose torque converter lockup in conjunction with automatic shift control for a change speed transmission.
A torque converter lockup control system for use on motor vehicles is disclosed in Miller et al. U.S. Pat. No. 2,646,864. This system provides for engaging lockup at a predetermined engine speed and for disengaging the lockup at a lower speed if the torque demand on the engine is high. The torque converter is locked up at low speeds when the torque demand is low. Under certain conditions, engine braking is obtained by converter lockup until low speed is reached by means of a holding coil energized by the vehicle generator.
In the Schneider U.S. Pat. No. 3,805,640, a drive train is provided with a hydraulic retarder which is engaged when going downhill in converter lockup with overspeeding. The retarder is kept engaged until the lockup clutch is disengaged.
A general object of this invention is to provide torque converter lockup control systems which overcomes certain disadvantages of the prior art.